1) Field of the Invention
The present invention relates to a technology for measuring load on a spring. More specifically, the present invention relates to technology for measuring the load on a leaf spring (suspension) that supports the magnetic head in a hard disk drive (HDD).
2) Description of the Related Art
In many cases it is necessary to measure the load on a spring. For example, it is necessary to measure the load on a suspension, which supports the magnetic head in the HDD.
In accordance with dramatic improvement in a recording density of the HDD, it has become necessary to accurately manufacture the suspension. An amount of levitation of the magnetic head at the time of recording and reproduction depends on how much load there is on the suspension in a stationary state. Consequently, a head load of the suspension significantly affects a levitation posture and a levitation characteristic of the magnetic head. Therefore, head load of each suspension is measured when manufacturing the suspension.
FIG. 18 is a schematic for explaining a relation between a suspension 200 of a magnetic disk 21. It is assumed here that the magnetic disk 21 is not rotating. The suspension 200 includes a support 24 that supports a base plate 201, and a load beam 203 that is attached to the base plate 201 via a leaf spring section 202. A flexure 209 is attached to the load beam 203.
A slider 210 is attached to an upper surface of the flexure 209. The slider 210 slides with respect to a surface of the magnetic disk 21. A magnetic read/write head (not shown) is housed inside the slider 210. A sliding surface of the magnetic head opposed to the magnetic disk 21 is the slider 210. A dimple 208a is provided at a tip of the load beam 203 and it is in contact with the flexure 209. The dimple 208a serves as a rotation fulcrum for the slider 210.
The load beam 203 is elastically supported by the leaf spring section 202. Therefore, when the magnetic disk 21 is not rotating, the slider 210 is pressed against the magnetic disk 21 due to the force of the leaf spring 202. The contact load, when the magnetic disk 21 is not rotating, with which the slider 210 is pressed against the magnetic disk 21 will be called the head load.
When the magnetic disk 21 rotates, the slider 210 is pushed away from the magnetic disk 21 because of an airflow that is generated because of the rotation of the magnetic disk 21. In other words, the slider 210 levitates below (or above) the magnetic disk 21. Recording and/or reproduction of information from/in the magnetic disk 21 is performed in this manner. The amount of levitation depends on the buoyant force and a force caused by the bending of the suspension. In general, this amount of levitation is several nanometers to several tens nanometers.
The head load has been conventionally measured by a method as described below. This method is disclosed, for example, in Japanese Patent Application Laid-Open Publication No. H6-44760.
Note that, in an example described below, an object of measurement of the heat load is the suspension 200 not yet mounted with the slider 210. When the suspension 200 mounted with the slider 210 is an object of measurement, it is possible to measure the head load with the same method except that only a thickness (Z210 in FIG. 19) of the slider 210 has to be taken into account.
In FIG. 19, reference numeral 200a denotes the suspension 200 at the time when it is free (no load state). Reference numeral 200b denotes the suspension 200 at the time when the slider 210 is in contact with the magnetic disk 21 at the time of rotation stop (the same state as FIG. 18). A load given to the magnetic disk 21 by the suspension 200b via the slider 210 is the head load.
Reference sign Zf denotes a height of a flexure 209a of the suspension 200a at the time when it is free from a reference plane 25a of the fixed support 24. Reference sign Z21 denotes a height of a lower surface of the magnetic disk 21 (a surface in contact with the magnetic disk 21) from the reference plane 25a. Reference sign Zh denotes a height of a flexure 209b of the suspension 200b, which presses the slider 210 against the magnetic disk 21 at the time of rotation stop, from the reference plane 25a. The height Z21 of the lower surface of the magnetic disk 21 is (Zh+Z210).
Therefore, in measuring the head load of the suspension 200, as shown in FIGS. 8 and 9, in a state in which a load probe 310 of a load cell 300 is in contact with the flexure 209, the load cell 300 only has to be lowered to depress the flexure 209 to a position of the height Zh and measure a load (reaction) from the flexure 209 in that state with the load cell 300. In the following description, a more specific method of measuring a head load will be explained with reference to FIG. 21:    (1) first, prepare a suspension 200M (master workpiece), a load of which is known in advance;    (2) nip the master workpiece 200M with a workpiece clamp 400 and press the master workpiece 200M upward to fix it on a reference plane 401;    (3) move a load cell 420 upward with a vertical movement unit 410 such as an air cylinder to bring a load probe 425 of the load cell 420 into contact with the master workpiece 200M;    (4) move up and down an ascending-end stopper 430 to adjust a height of the load cell 420 while monitoring a load outputted by the load cell 420, and fix the ascending-end stopper 430 at a position where the load outputted by the load cell 420 coincides with the known load of the master workpiece 200M (completion of the adjustment of a height of the load cell 420);    (5) lower the load cell 420 and also lower the workpiece clamp 400 to release the master workpiece 200M;    (6) fix the suspension 200, which is the object of measurement, on the reference plane 401 with the workpiece clamp 400;    (7) lift the load cell 420 to bring it into abutment against the ascending-end stopper 430 which has been adjusted in (4) above; and    (8) obtain a load outputted by the load cell 420 in the state of (7) as a head load of the suspension 200.
When the load of the suspension 200 is measured as described above, a result of the measurement is compared with a proper load to correct the suspension 200 such that the proper load is obtained. In recent years, a technique for irradiating a laser beam on the suspension 200 and utilizing thermal deformation of an irradiated portion to correct a load has been proposed.
As such a correction technique, a technique disclosed in Japanese Patent Application Publication No. H7-77063 is known. In this technique, a laser beam is irradiated in a width direction of a spring arm to correct a load such that a specified contact pressure is obtained.
However, the conventional technique does not give accurate results. In particular, in a suspension that supports a magnetic head, accuracy required in measurement of a head load is extremely high. Whereas a load required of the suspension was about 3±1.5 grams-force (gf) or 2.5±0.4 gf in the past, the load is measured at accuracy of as high as, for example, 0.4±0.04 gf recently.